Like the road trip upon which the titular character embarks, Tammy isn’t quite sure of its ultimate destination.

More broadly comic than many dramas but punctuated with dramatic moments out of character for a mainstream comedic film, this Melissa McCarthy vehicle is another frustrating exercise in one of Hollywood’s funniest people being squandered in a project unworthy of her skills.

That McCarthy, along with her husband, Ben Falcone, is responsible for the screenplay is doubly mystifying — are quality comedy scripts that hard to come by?

Tammy also has the misfortune of feeling familiar before the story really gets rolling.

McCarthy has carved out a niche among comedians with her willingness to play characters that are rough around the edges but with a core sweetness leavening any coarseness.

It’s evident from the opening scene — involving an unfortunate fender bender with a woodland creature — that Tammy, like McCarthy’s roles in Identity Thief, The Heat and even all the way back to her breakout turn in Bridesmaids, is decent, if somewhat crude and prone to speaking bluntly.

After a spectacularly awful day, during which Tammy is unceremoniously fired from her fast-food job and arrives home to find her husband, Greg (Nat Faxon), enjoying a romantic dinner with the neighbor Missi (Toni Collette), she enlists her rebellious, alcoholic grandmother Pearl (Susan Sarandon) for a spontaneous trip to Niagara Falls, becoming enmeshed in all manner of outrageous shenanigans along the way.

Such travel-oriented narratives are episodic by design, which allows Tammy some freedom — perhaps too much.

Tammy quickly realizes Pearl is going to be a handful, as Pearl aggressively pursues Earl, an amorous farmer (Gary Cole), which leads to an improvised robbery of the same fast-food chain that fired her, a brush with possible love (in the form of Mark Duplass) and a “lesbian Fourth of July party,” capped with a cringeworthy toast and the image of Kathy Bates lighting a personal watercraft on fire.

All the while, Tammy’s mother, Deb (Allison Janney), is making endless worried phone calls, concerned about not only Tammy but her mother’s failing health.

It’s hard to know whether Tammy is intended as a bid for McCarthy to assume more dramatic responsibilities — to this point, she’s almost exclusively starred in big-screen comedies — but in the few genuinely dramatic scenes here, the actress acquits herself well, mining real pathos from an unlikable character.

There’s a pivotal scene, late in the film, involving Pearl and Tammy at a lakeside home that — without giving anything away — is a fine piece of emotionally charged acting, albeit one almost immediately undermined by an easy gag.

That tension between what McCarthy has become known for (wince-inducing pratfalls; characters with no filter) and what she’s capable of (giving these human cartoons some shading and depth) makes Tammy seem compromised in a way, a concession to box-office realities rather than seizing an opportunity to put a fresh spin on a comic summer blockbuster.

McCarthy and Falcone (making his directorial debut here, as well as co-producing and starring as Tammy’s skeevy boss) are avowed improv enthusiasts, and several scenes feel like the performers are simply standing around, waiting for inspiration to strike.

A few gems emerge from the ether, but far too often, Tammy slows to a crawl as funny people stare at one another.

Such inertia is baffling and gives the impression that not much of the film was sketched out beyond Tammy’s metaphorical journey.

The supporting characters are little more than archetypes — the gruff but lovable father, played by Dan Aykroyd; the goofball husband; the hesitant but pure-hearted love interest; the hectoring but ultimately committed mother — giving the entire film a sense of familiarity that tips over into homogeneity.

Tammy is fitfully amusing, but once again, the joke’s on Melissa McCarthy. What could have been an excursion into uncharted territory ends up instead feeling like another dead end.

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